The C. elegans postembryonic mesoderm (M) lineage is derived from a single pluripotent precursor, the M mesoblast, which will produce 32 cells, including 14 bodywall muscles (BWMs), two coelomocytes (CCs), and two sex myoblasts (SMs) that produce 16 sex muscles. SEM-4, the sole C. elegans member of the SALL family of C2H2 zinc finger transcription factors is required for both CC and SM fates, as both are transformed to BWM cells in
sem-4 null mutants (Basson and Horvitz, 1996). Here we investigate the role of SEM-4 during the development of the M-lineage. Based of analysis of additional
sem-4 mutants, only a subset of highly conserved zinc fingers (5-7) seems to be required for the M-lineage phenotypes. Indeed,
sem-4 is expressed in the M-lineage precursors that will give rise to both the CCs and SMs as well as the SM descendants. This restricted expression requires input from M lineage intrinsic factors, such as FOZI-1, and SYS-1/beta-catenin, which is downstream of the Wnt/beta-catenin asymmetry pathway. It also requires the presence of large intronic sequences in the
sem-4 locus. Forced expression of SEM-4 throughout the M-lineage is able to produce extra SMs, suggesting that while
sem-4 is necessary for both the CC and SM fate, it is sufficient to specify the SM fate. The mis-expression phenotype of
sem-4 is similar to the loss-of function phenotype of
mls-2/Hmx mutants, suggesting that MLS-2 might negatively regulate
sem-4. However, analysis of
sem-4;
mls-2 double mutant phenotypes did not suggest a simple epistatic relationship between the two genes. The M lineage phenotype and expression pattern of
sem-4 regarding SM development resemble those of
sem-2/SoxC mutants, suggesting a a potential interaction. Results from molecular epitasis and yeast-two hybrid experiments suggest that
sem-2 and
sem-4 do not regulate each other's expression in the M-lineage or physically interact with each other. However,
sem-2 is required for SEM-4 to convert additional cells to the SM fate when mis-expressed. We are currently testing how SEM-4 and SEM-2 may interact to correctly specify the SM fate, what SEM-4 interacts with to specify the CC fate, and what are downstream of SEM-4 in both fate specification events.